Update PowhegControl user documentation

The Powheg-for-ATLAS user documentation has become outdated in many parts, to the point where even the basic usage wasn't properly documented anymore. This merge request remedies that.

Many advanced features still need updates to their documentation, which will come soon. We think that the most important thing is to fix the basic usage instructions as soon as possible. For those parts that still need updating/fixing, the documentation is now clearly marked as to be revised and the users are advised to contact the experts if they need features with outdated documentation.